1. Technical Field of the Invention
The present invention relates generally to an optical isolator, and more particularly to a polarization independent optical isolator which may be employed in optical fiber communication systems.
2. Background Art
It is well known in the art that a polarization independent optical isolation operable to block transmission of an optical signal in a reverse direction is an essential device for optical fiber communication systems. Such a polarization independent optical isolator usually utilizes magnetooptic effects which rotate the plane of polarization of incident light irreversibly.
U.S. Pat. No. 4,893,890 to Lutes, issued on Jan. 16, 1990, discloses a conventional optical isolator which includes a first birefringent plate for splitting incident light into two linearly polarized rays whose planes of polarization are oriented in mutually perpendicular directions, a magnetooptic plate for irreversibly rotating the planes of polarization of the linearly polarized rays about 45 deg in a counterclockwise direction, a half wave plate for irreversibly rotating the linearly polarized rays approximately 45 deg, and a second birefringent plate for recombining the two linearly polarized rays on the same optical axis. These elements are arranged in tandem.
In general, the degree of interception of light traveling through an optical isolator in the reverse direction, or the isolation characteristic is dependent upon characteristics of a magnetooptic plate. For example, a single-way isolation optical isolator using a single magnetooptic plate such as that taught by Lutes commonly exhibits the isolation characteristic on the order of 35 dB. This isolation characteristic is insufficient for applications to high-quality optical transmission systems or optical fiber amplifiers. Additionally, the prior art optical isolator is coupled at its both end surfaces to optical fibers through lenses in alignment with the optical fibers. Such an optical isolator encounters a drawback in that a large amount of arrangement space for optical fibers is required for mounting the optical isolator in optical communication systems.